I. Field of the Invention
The present invention relates to photodiodes within medical imaging systems; more particularly, the present invention relates to silicon photomultipliers (SiPMs) in medical imaging systems.
II. Background Information
The standard photo detector used in positron emission tomography (PET) applications is the photomultiplier tube (PMT). The high gain, fast response and high sensitivity of PMTs have made them a viable detector for PET, but there exist several drawbacks. One drawback is the bulky size of PMTs, which leads to large overall detector size and can put a limit on the spatial resolution of a detector. PMTs are also highly sensitive to magnetic fields, making it impossible to implement PET with magnetic resonance imaging when PMTs are utilized as the photo detector.
A relatively new photo detector, SiPM, is well suited for PET applications. It has similar sensitivity and gain to the industry standard photomultiplier tube (PMT), but has two important advantages, smaller size and insensitivity to magnetic field. SiPMs are semiconductor photon sensitive devices made up of an array of very small Geiger-mode avalanche photodiode (APD) cells or SiPM microcells on a silicon substrate, wherein the APDs within each respective cell are connected in parallel. Each SiPM microcell is an individual photon counter and the sum of all the SiPM microcells is the output of the SiPM.
SiPM microcells vary in dimension from 10 to 100 microns depending on the mask used, and can have a density of up to 10000 per square mm. Avalanche diodes can also be made from other semiconductors besides silicon, depending on the properties that are desirable. Silicon detects in the visible and near infrared range, with low multiplication noise (excess noise). Germanium (Ge) detects infrared to 1.7 μm wavelength, but has high multiplication noise. InGaAs detects to a maximum wavelength of 1.6 μm, and has less multiplication noise than Ge. InGaAs is generally used for the multiplication region of a heterostructure diode, is compatible with high-speed telecommunications using optical fibers, and can reach speeds of greater than Gbit/s. Gallium nitride operates with UV light. HgCdTe operates in the infrared, to a maximum wavelength of about 14 μm, requires cooling to reduce dark currents, and can achieve a very low level of excess noise.
Silicon avalanche diodes can function with breakdown voltages of 100 to 2000V, typically. APDs exhibit internal current gain effect of about 100-1000 due to impact ionization, or avalanche effect, when a high reverse bias voltage is applied (approximately 100-500 V in silicon). Greater voltage can be applied to silicon APDs, which are more sensitive, compared to other semiconductor photodiodes, than to traditional APDs before achieving breakdown allowing for a larger operating gain, preferably over 100, because silicon APDs provide for alternative doping. Gain increases with reverse voltage, and APD gain also varies dependently on both reverse bias and temperature, which is why reverse voltage should be controlled in order to preserve stable gain. SiPMs can achieve a gain of 10.sup.5 to 10.sup.6, by operating with a reverse voltage that is greater than the breakdown voltage, and by maintaining the dark count event rate at a sufficiently low level.
Geiger-mode APDs produce a large, fast pulse of the same amplitude when struck by a photon no matter the energy of the photon. When many of these cells are placed together in an array, they can be combined into one large parallel circuit which will produce an output pulse proportional to the input photon pulse. This device is referred to as a SiPM. However, as the size of the array increase, so does the input capacitance to an amplifier and noise of the device. The supply voltage needed for a SiPM device varies from 30V to 100V depending on the junction type, and is less than the supply voltage needed for a PMT by a factor of from 10 to over 40. The capacitance and noise are proportional to the area of the SiPM device. The rise time of the device also increases with its capacitance, and the rise time and noise are major factors in determining the time resolution in PET. The timing resolution degrades if the rise time becomes longer and the signal becomes noisier. Therefore, the optimal SiPM device would be a very small, fast, low noise device. However, the smaller the device is, the fewer photons that can be collected to be used for the 511 keV energy discrimination. Thus, typically the size of the device needs to be compromised, resulting in a device that is as large as needed for adequate light collection and energy resolution.
The primary requirements for a photosensor for PET detection are high photon detection efficiency (PDE), low noise level and very high trigger time accuracy. These are the key parameters that influence the time resolution and energy resolution of the measured gamma events. In addition to these properties, the sensor also needs to be compatible with magnetic fields, if it is to be used for combined PET/MR imaging.
Developers have found that SiPMs show the most promising results for an MR-compatible detector that displays high energy and timing resolution. Notwithstanding, developers have found it to be a challenge to develop a SiPM-based detector that provides the same time resolution as the PMT-based block detectors used in modern time of flight (TOF)-PET scanners. There is a need for a SiPM-based detector that overcomes the problems associated with increasing the size of the active area of SiPM arrays.